This invention relates to the removal of solder films from printed circuit boards and in particular, to a new and improved method and composition for stripping the solder film and the underlying tin-copper alloy from the copper substrate of a printed circuit board in a single application of the composition, as by spraying or dipping, without the formation of sludge.
A typical printed circuit board has a copper conductor pattern on an insulating support. Solder is applied onto the copper substrate, typically by electroplating. The solder film typically is in the order of 0.0003 inches thick which is the standard thickness used in most present day processes. This is a nominal value, for the thickness may vary considerably over the board and from board to board. After the solder film is applied onto the copper, a thin film of copper-tin alloy forms between the copper and the solder, typically about 0.000002 to 0.000004 inches thick. This copper-tin alloy film increases in thickness with time.
As used in the specification and claims, the word "solder" includes the various tin-lead alloys and substantially pure tin as used in providing films on printed circuit boards. Such films are produced by various methods, including electroplating, chemical deposition and immersion in a melt. While the 60-40 tin-lead solder film typically is about 0.0003 to 0.0004 inches thick, the pure tin film typically is about 0.0002 inches thick. The film is used as a resist over the copper during etching of the copper in the production of solder mask over bare copper boards.
In the manufacturing process the solder film is stripped from the copper substrate. Two types of compositions have been used for this solder stripping in the past. One composition is an acid solution of hydrogen peroxide and a fluoride. The other stripper is a nitric acid solution inhibited so that it will not attack copper. In general, the nitric acid based solder strippers are superior to the peroxide-fluoride based strippers, and have been used commercially for a number of years.
The peroxide-fluoride compositions are undesirable because there is an exothermic reaction during the stripping which heats the solution to a temperature which decomposes the unstable peroxide and makes the solution unusable. Hence the solution requires cooling during use. Also, the peroxidefluoride solutions attack the epoxy fiberglass substrate and are very difficult to waste treat because of their chelating/complexing properties and toxicity. Nitric acid based solutions do not exhibit these problems.
In earlier conventional nitric acid stripper systems, the nitric acid solution is maintained in a tank and the board is immersed in the solution for a period of time required to dissolve the solder. Following removal of the solder, it is necessary to remove the tin-copper alloy layer to expose the copper substrate. This is achieved by then immersing the board in a second tank containing another solution which dissolves the tin-copper alloy and ordinarily a small amount of the pure copper substrate. This second solution may be for example, ferric chloride, ammonium persulfate, peroxide sulfuric acid, or peroxide-fluoride.
The basic composition and method for single bath and spray stripping is now well described in prior art patents. A number of results are desired in the solder stripping operation, including complete stripping in a minimum time, a resultant bright surface on the exposed copper, a minimum of foaming and fuming, all without the formation of sludge in the stripper tank. It has not been possible to achieve all these results with the prior stripping compositions, and it is an object of the present invention to provide a new and improved composition and method for stripping which provides complete stripping with a resulting bright surface, in a short time, without foaming or fuming, and substantially without formation of sludge which needs to be removed before charging the process tank with fresh solution.
The composition of U.S. Pat. No. 4,713,144 utilizes a combination of nitric, ferric and sulfamic acid which operates satisfactorily for stripping in a relatively short time, but did result in relatively high tin rich sludge. Formulations containing organic acids have been tried to solve the sludge problem but were not satisfactory in that they only delayed the sludge formation and at the same time were very unstable. The organic acid bearing strippers yielded colloidal sludge in the absence of sulfamic acid but did not perform well in the presence of sulfamic acid. Also, various inhibitors have been tried in an effort to improve performance. By way of example see Japanese patent disclosure 58 (1983)-58280 and U.S. Pat. No. 4,921,571 and patents discussed therein.
Another known formulation avoids the formation of sludge but does not provide a clean bright copper surface, while at the same time producing fumes which have to be taken care of.
It is an object of the present invention to provide a new and improved stripping composition and method of stripping which utilizes only a single composition and a single application step and which achieves the desired bright surface of the cleaned copper, while substantially eliminating the production of the undesirable sludge which accompanies prior stripping operations.
By way of example, in the past it usually has been necessary to shut down the stripping operation after about two days in order to remove the accumulated sludge from the stripping tank. With the composition of the present invention, it is now possible to operate the same equipment for two weeks without requiring shut down, and in some situations the equipment can be operated continuously using the feed and bleed method of maintaining composition strength, without interrupting operation for sludge removal.
It is a further object of the invention to provide such a stripping composition and method which can utilize conventional inhibitors for reduction of fumes and reduction of foaming, without adversely affecting the operation or life of the basic composition.
These and other objects, advantages, features and results will more fully appear in the course of the following description.